The present invention relates to a gear train structure adapted for use in electronic watches.
While the recent trend is toward increasing the accuracy of electronic watches, the method of intermittent hand movement at intervals of a second or so has been employed as the hand movement method for the watches of the analog type and the occurrence of an indication deviation for every hand movement has been a problem. This has been due to the backlash between the fourth wheel to which the second hand is attached and the other wheel in mesh with the fourth wheel and the supporting structure of the hand wheels.
As a conventional method of preventing an indication deviation due to such backlash, there has been known the method of holding down the forward end of the upper tenon of the fourth wheel in the axial direction by a restraining spring so as to apply a restraining force. FIG. 7 of the accompanying drawings is a diagram useful for explaining the conventional indication deviation preventing method shown for example in Japanese Utility Model Registration Publication No.63-46862. As shown in FIG. 7, the conventional indication deviation preventing method is so designed that a restraining spring 55 presses the forward end of the upper tenon of a fourth wheel 53 to which a second hand 50 is attached and the resulting pressing force prevents any tottering or unsteady movement of the second hand.
However, the upper tenon forward end 53a of the fourth wheel 53 is pointed to have a conical shape to improve the assembly performance of the gear train bridge thus giving rise to a problem that the restraining spring 55 is caused to wear due to its contact with the upper tenon forward end and its durability is deteriorated. There is another problem that the powder caused by the wear enters the gap between the upper tenon portion of the fourth wheel 53 and the tenon guide of the gear train bridge 57, thereby preventing the rotation of the fourth wheel 53 and hence causing the watch to stop.
Also, with the gear train structure of the ordinary electronic watch, as shown in FIG. 7, the rotation of a rotor 59 (a sixth wheel) of a motor is transmitted to the fourth wheel 53 through a fifth wheel 61 and thus the load applied to the fourth wheel 53 has a great influence on the motor efficiency. In other words, if the pressing force of the restraining spring 55 is excessively large, the load applied to the motor is increased and the current consumption is increased thereby reducing the life of the motor.
On the contrary, if the pressing force is excessively small, the tottering of the second hand cannot be reduced to a minimum.
Thus, there is a problem that the pressing force of the restraining spring 55 must be adjusted to such magnitude that no ill effect is produced on the motor efficiency and moreover the tottering of the second hand is reduced to a minimum and that this adjustment is extremely difficult. Where it is desired to vary the load on the gear train, the spring shape or the amount of deflection of the restraining spring are conventionally set anew. However, it is difficult to accurately form the desired spring shape and it is rather difficult to realize the aimed load.
In addition, while the restraining spring 55 applies an axial force to the fourth wheel 53 so that any tottering of the second hand immediately after the wheel movement is reduced somewhat by the axial load, it is impossible to control the angle of rotation of the second hand. In other words, there still exists a problem that the restraining spring 55 is not capable of going to the extent of controlling the indicating position of the second hand and the indicating position is varied thus causing a deviation in the indication of the second hand.
On the other hand, the following two types have heretofore been used as the supporting structures for the hand wheels of the watches in which the three hands including the hour, minute and second hands are arranged concentrically. In other words, as shown in FIG. 8. The first structure is so constructed that a second wheel shaft 65 is secured to a second gear train bridge 63 and a fourth wheel 53 or a second indicating wheel and a second wheel 67 or a minute indicating wheel are respectively rotatably supported by the inner and outer peripheral surfaces of the second wheel shaft 65, and the second structure is so designed that an hour wheel 69 or an hour indicating wheel and a second wheel 67 of a minute indicating wheel are separately arranged and rotatably supported on the inner and outer peripheral surfaces of a center pipe (not shown) fixedly mounted in a base plate.
However, the above-mentioned two structures respectively have the following problems. More specifically, in the case of the first structure (the one including only the second wheel shaft), while the fourth wheel 53 and the second wheel 67 are completely separated from each other in a non-contact manner by the second wheel shaft 65, the hour wheel 69 or the hour indicating wheel is supported by the outer periphery of the second wheel 67 so that variation in the plane position of the hour wheel 69 includes variation in the plane position or play of the second wheel 67 supporting the former and therefore the deviation in the plane position of the hour wheel 69 is increased. Thus, there results an increase in the variation in the extent of engagement of the toothed portion of the hour wheel 69 thus causing an increase in the amount of backlash and hence an increase in the indication deviation of the hours hand fitted on the hour wheel 69.
Also, in the case of the second structure (the one including only the center pipe), while the hour wheel 69 and the second wheel 67 are completely separated by the center pipe, the minute indicating wheel or the second wheel 67 and the second indicating wheel or the fourth wheel 53 are always in contact and therefore there is the danger of causing the minute hand to jerk in association with the movement of the second hand 50.
Also, the prerequisite of a highly accurate watch requires a condition that the watch is not caused to stop and it is the usually practice with the plane layout of the conventional movement parts (the component parts of the watch excluding the watch case and the battery are referred to as movement parts) to arrange no other component parts or the like between the battery pocket and the gear train pocket thereby interconnecting the battery pocket and the gear train pocket through a space.
With the construction in which no partition is provided between the battery pocket and the gear train pocket as mentioned above, however, there is a problem that when changing the batteries, dust, nap or the like tends to enter through the battery pocket and such dust, nap or the like tends to impede the movement of the gear trains thereby leading to the stoppage of the watch.